Triangular nanoframes and method of making same

ABSTRACT

The present invention provides nanoprisms etched to generate triangular framework structures. These triangular nanoframes possess no strong surface plasmon bands in the ultraviolet or visible regions of the optical spectrum. By adding a mild reducing agent, metal ions remaining in solution can be reduced, resulting in metal plating and reformation of nanoprisms. The extent of the backfilling process can be controlled, allowing the formation of novel nanoprisms with nanopores. This back-filling process is accompanied by a regeneration of the surface plasmon bands in the UV-visible spectrum.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/454,552 filed Mar. 14, 2003, which is incorporated herein inits entirety by this reference.

FIELD OF THE INVENTION

The invention resides in the field of nanoparticles, particularlytriangular nanoframes that may be backfilled to form nanoprisms havingnanopores.

BACKGROUND OF THE INVENTION

Metallic nanoparticles have generated significant scientific andtechnological interest due to their unusual optical properties, as wellas their novel chemical and catalytic properties. Nonsphericalparticles, and in particular anisotropic particles, are of majorinterest because they allow investigation of how shape affects thephysical and chemical properties of such structures. A variety ofshapes, including stars, cubes, rods, discs, and prisms, have beenfabricated, and their properties have been preliminarily characterized.Hollow nanoparticles are an interesting emerging class of materials thatwill help to better understand the structure-property relationship innanoparticles.

Although a significant amount of work has been done in developingsynthetic methods for hollow spheres, cubes, and rods, little has beendone with triangular nanostructures. In copending patent application(Publication No. 20030136223) the current inventors disclosed a novelprocess in which silver nanospheres were converted, via a photomediatedreaction, to larger silver nanoprisms. Xia and coworkers havedemonstrated the production of hollow forms of cubes and rods, but havenot disclosed a method of making frames from nanoprisms.

Thus, there remains a need for a method of generating triangularnanoframes. Preferably, the method would be operative on nanoprismsformed by known methods and applicable in a face-selective mannerallowing the generation of triangular two-component nanostructures withfilled or partially-filled cores.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a triangularnanoframe including the step of etching a nanoprism with a salt to forma nanotriangle. The nanoprism may be silver and the salt may be a metalsalt such as HAuCl₄. Preferably, the triangular nanoprism is contactedwith the salt in a suspension of nanoprisms to which the salt is addeddropwise.

The present invention also provides a method of narrowing or closing thepore in the triangular nanoframe by contacting the nanoframe with areducing agent in the presence of the salt. Using this method, the wallsof the nanoframe can be thickened to leave a narrow pore through thenanoframe. Typically, the pore has a diameter of less than about 35 nmand preferably between about 4 nm and about 14 nm. The thickness of thenanoframe is typically between about 10 nm and about 15 nm. The reducingagent is preferably a mild reducing agent such as ascorbic acid. Thenanoframe may be repeatedly contacted with the salt to progressivelythicken the walls of the nanoframe and reduce the diameter of the pore.If the contact with the reducing agent in the presence of a salt isrepeated several times, the method of the present invention results inthe reproduction of nanoprisms.

The present invention also provides triangular nanoframes having an edgelength of less than about 200 nm and a thickness of less than about 100nm and a pore through the center of the nanoframe. The triangularnanoframes have an edge length between about 70 nm and about 80 nm, athickness between about 5 nm and about 15 nm, and a pore size betweenabout 5 nm and about 35 m.

The present invention also provides triangular nanoframes made by theprocess of etching a nanoprism with a salt to form a nanotriangle. Inthis embodiment, the nanoprism is preferably a silver nanoprism and thesalt is preferably HAuCl₄.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scheme of nanoframe synthesis. In step A, silvernanoprisms are etched with aqueous HAuCl₄. Subsequent addition ofL-ascorbic acid (step B) causes gold and silver ions in solution tocrystallize primarily on the inner walls of the nanoframes, causing thecentral pore to shrink in size. This gold salt/L-ascorbic acid cycle(Steps A+B) can be repeated to progressively shrink the size of thecentral pore.

FIG. 2 shows gold-silver nanoframes; A) UV-visible spectra of triangularnanoframes with varying Au:Ag ratios and B-D) tunneling electronmicroscope (TEM) images of gold-silver nanoframes; B: Au:Ag ration of1:9; C: Au:Ag ratio of 1:5; D: Au:Ag ratio of 1:3.

FIG. 3 shows UV-visible spectra and TEM images monitoring theback-filling process of silver and gold nanoframes having a Au:Ag ratioof 1:9. A,D) After addition of L-ascorbic acid to the triangularnanoframes. B,E) After 2 cycles of HAuCl₄/L-ascorbic acid. C,F) After 3cycles of HAuCl₄/L-ascorbic acid.

DETAILED DESCRIPTION OF THE INVENTION

The methods of the present invention produce triangular nanoparticleswith a hollow center resembling a nanotriangle. The methods of formingtriangular nanoframes include etching a nanoprism with a salt to form ananotriangle. The nanoframes may be exposed to a reducing agent in thepresence of the salt causing backfilling of the hollow center of thenanoframe and thickening of the walls of the nanoframe. Repeatedexposure to a reducing agent in the presence of a salt may progressivelybackfill the entire hollow cavity of the triangular nanoframe to form asolid triangular structure or reform a nanoprism.

The methods of the present invention form a new class of nanostructures,metallic triangular nanoframes. Additionally, the etching isface-selective and the etching and novel back-filling process results inthe formation of triangular nanoframes and prisms with differentcompositions. Also, in a preferred embodiment, these synthetic methodsresult in the conversion of silver nanoprisms to gold-silver alloynanoprisms, which are otherwise not accessible via thermal andphotochemical methods for making monometallic nanoprisms.

The methods of the present invention take advantage of the largedifference in reduction potential of the two reactants—the moleculesforming the nanoprisms and the salt. Preferably, the nanoprisms are ametal such as silver and the salt is a metal such as gold. The disparityin the reduction potential of the Ag+/Ag pair (0.8 V, vs. SHE) andAuCl⁴⁻/Au pair (0.99V, vs SHE), results in the oxidization and etchingof the silver nanoprisms by gold ions in a type of nano-Galvanic cellreaction. Surprisingly, this method does not yield hollow nanoprisms, aswould be expected using the techniques of the prior art, but rathergenerates triangular-shaped frames with solid walls and a hole in thecenter (FIG. 1, Step A). Triangles are formed because this etchingapproach is selective for the [ 111] faces of the silver nanoprism overthe [ 110] crystal plane that makes up the edges. Without intending tobe bound by any one theory, this may be due to the fact that the initialnanoprism particles have thickness of only about 10 nm or that thereaction conditions are milder than those of the prior art methodology.Additionally, these structures can be backfilled to generate nanoprismsincorporating the components of the nanoframe and the salt. In theinstance of a silver nanoprism and a gold salt, the nanoframes can bebackfilled with gold to generate gold-silver alloy nanoprisms. Notably,these etching processes of the present invention are not observed withall metal ions. For example silver nanoprisms are not etched in aH₂PtCl₆ salt solution, possibly due to the relatively large latticemismatch between platinum and silver (Pt=3.9231 Å, Ag=4.0862 Å).

The nanoprisms which represent the starting materials for the methods ofthe present invention have an edge length of less than about 200 nm andpreferably less than about 100 nm. More preferably, these nanoprismshave an edge length of less than about 80 nm and most preferably have anedge length of between about 70 nm and about 80 nm. These nanoprismshave a thickness of less than about 100 nm and preferably less thanabout 25 nm. More preferably, these nanoprisms have a thickness of lessthan about 20 nm and most preferably have a thickness of between about 5nm and about 15 nm. The nanoprisms may be composed of any substance thatis effectively etched by the salt. Typically, the nanoprism is a metalnanoprism and preferably the nanoprism is a Group VIII, IB or IIB metaland most preferably, the nanoprism is a silver nanoprism. The nanoprismsmay be made by any suitable method and methods of making metalnanoprisms suitable for use in the methods of the present invention areknown in the art (Jin et al. Science 294:1901 (2001); co-pending U.S.patent application 20030136223).

The nanoprisms are preferably diluted in an aqueous solution to aconcentration that will prevent agglomeration of the nanoframes formedin the methods of the present invention. Typically, the nanoprisms aresuspended in water at a concentration between about 1 M and about 30M.Preferably, the nanoprisms are diluted to a concentration of betweenabout 15 M and about 20 M prior to contact with the reducing agent.

The nanoprisms are contacted with a salt to etch the nanoprism to formnanoframes. The salt may be any suitable salt with a greater reductionpotential than the composition of the nanoframes. Typically, the salt isa metal salt, and preferably the salt is a Group VIII, IB or IIB metalsalt and most preferably, the salt is a gold salt such as HAuCl₄. Thiscontact is best initiated by slow or dropwise addition of the salt tothe nanoprism suspension, preferably with rapid stirring of thenanoprism suspension. The amount of salt to add to the suspension ofnanoprisms should be calculated by the ratio of the reductant within thesalt and the chemical composing the nanoframes. For example, in theembodiment of the present invention in which silver nanoprisms areetched with HAuCl₄, the gold salt should be added to achieve a ratio ofgold to silver (Au:Ag) between about 1:2 to about 1:10. Preferably, theAu:Ag is about 1:5. In many instances, the gold content of thesuspension can be monitored by the color of the resulting suspension.For example, in the instance of silver nanoprisms etched with HAuCl₄, asthe gold salt is added, the turquoise-blue color of the silver nanoprismcolloid gradually changes to purple and finally to blue or grey. Sampleswith low gold content (Au:Ag=1:9) form pale blue solutions and exhibit alow intensity, broad surface plasmon band around 775 nm. In contrast,colloids containing high gold concentrations (Au:Ag=1:5 to 1:3) are palegrey (essentially colorless) and display no strong surface plasmon bandsin the UV-visible spectrum (FIG. 2A).

Without intending to be bound by any one theory, it is believed thatbecause nanoprisms possess well-defined crystal faces (instead of thehighly faceted surfaces typical of “spherical” nanoparticles), the saltetches in a face-selective manner in which the prism face ([111] crystalplane) is selectively oxidized over the nanoprism edges ([110] planes).This explanation would account for the retention of the triangular shapeof the initial silver nanoprisms while the gold salt etches the centralsilver matrix. Transmission electron microscopy (TEM) images after saltaddition confirm that the resulting nanostructures are triangular inshape with hollow centers (FIG. 2B-D). The wall width of the triangularnanoframes formed refers to the width of the nanoframe vertice (whenviewed from above) whereas the thickness of the triangular nanoframesrefers to the height of the particle (perpendicular to its longestdimension). The pore in the triangular nanoframes refers to the passagethrough the center of the nanoframe and the pore size refers to thediameter of that hole. The wall width of the nanoframe increasesslightly with the increasing content of the salt in the suspension. Thethickness of the nanoframes is similar to that of the nanoprisms fromwhich they are derived.

In another embodiment of the present invention, methods of filling thehollow center of the triangular nanoframes have been devised to changethe size of the central pore (FIG. 1, Step B). In this embodiment, thenanoframes are contacted with a mild reducing agent. The reducing agentcauses materials removed from the nanoprisms to be reduced andagglomerate once again to the nanoframe. This causes the walls of thenanoframes to thicken and reduces the size of the central pore. As shownin FIG. 3, subsequent additions of the salt followed by the reducingagent (FIG. 1, Steps A and B), are performed to progressively reduce thesize of the pore in the triangular nanoframes. Typically, the pore sizeis in the range between about 5 nm and about 35 nm. Referring to FIG. 3,note how the pore size becomes gradually smaller with an increasingnumber of cycles. Part of the material added back to the nanoframe caninclude the salt originally used to etch the nanoprism. Any mildreducing agent is suitable for use in the present invention. Examples ofsuitable reducing agents include sodium formaldehyde sulfoxylate,2-mercaptoethanol, cysteine hydrochloride, sodium thioglycolate,hydroquinone, p-aminophenol and ascorbic acid. An exemplary reducingagent is ascorbic acid as this chemical is a mild reducing agent,nontoxic, and inexpensive. The reducing agent is added in excess,preferably dropwise to the stirring suspension of nanoframes. Forexample, in the instance of silver nanoframes etched with HAuCl₄,successive additions of ascorbic acid and HAuCl₄ results in a gold andsilver alloy triangular nanoframe with a partially or completely filledpore. As the pore closing occurs, the metal ions (gold and silver) seemto primarily crystallize on the inner walls of the nanoframe. The facesand edges of the gold nanoprisms are rough in texture and their cornersare truncated. The average edge length of the gold-silver alloynanoprisms is between about 30 nm and about 80 nm. Typically, theaverage edge length of the gold-silver alloy nanoprisms is about 60 nm.

The UV-visible spectrum of the filled nanoframes can be monitored toreview the progress of the pore closing. In the case of silvernanoframes, the UV-visible spectrum is red-shifted and dampened withrespect to the pure silver nanoprisms (FIG. 3C). This is the samephenomenon observed in spherical Au—Ag alloy nanoparticles in which thesurface plasmon band of silver nanoparticles is redshifted and dampenedwith increasing amounts of gold. Hence, this UV-visible data confirmsthat Au—Ag alloy nanoprisms are formed.

EXAMPLES Example 1

This example demonstrates the production of silver triangularnanoframes. Silver nanoprisms having an edge length of about 74 nm(σ=13%, N=200) and a thickness of about 9 nm (σ=27%, N=46) were preparedas described previously (U.S. patent application Publication No.20030136223 incorporated herein by this reference). The silver nanoprismcolloid was synthesized from a 0.1 mM AgNO₃ solution. The Au:Ag molarratios were calculated assuming that the silver concentration was 0.1mM. 10 mL of silver nanoprisms were diluted with pure water to one-fifththe starting concentration (18.2M). This was done to prevent aggregationof the resulting Ag/Au triangular nanoframes. Under ambient conditions,aqueous HAuCl₄ (5 mM) was added dropwise to the rapidly stirringcolloid. As the gold salt was added, the turquoise-blue color of thecolloid gradually changed to purple and finally to blue or gray. Sampleswith low gold content (Au:Ag=1:9) formed pale blue solutions andexhibited a low intensity, broad surface plasmon band around 775 nm. Incontrast, colloids containing high gold concentrations (Au:Ag=1:5, 1:3)were pale gray (essentially colorless) and displayed no strong surfaceplasmon bands in the UV-visible spectrum (FIG. 2A). Transmissionelectron microscopy (TEM) images after gold addition showed that theresulting nanostructures were triangular in shape with hollow centers(FIG. 1B-D).

Both wall width and thickness were measured. The wall width of thenanoframes increased slightly with gold content; 7.7 nm (σ=11%, N=245)for Au:Ag=1:9 and 10.3 nm (σ=21%, N=230) for Au:Ag=1:3. The thickness ofthe gold-silver nanoframes (10 nm, σ=20%, N=24) was similar to that ofthe pure silver nanoprisms starting materials (9 nm). TEM analysis athigh magnification (200,000×) revealed that the center of each nanoframewas indeed hollow, the amorphous carbon film of the TEM support gridcould be clearly seen in the underlying area. Tapping mode atomic forcemicroscopy (AFM) analysis (using a Nanoscope III AFM, DigitalInstruments) also confirmed that these nanoframes were hollowstructures.

Example 2

This example demonstrates protocols that control the optical propertiesof the nanoframes, by changing the size of the central pore. A mildreducing agent, L-ascorbic acid, was used to reduce gold and silver ionsin solution (generated from the first addition of gold salt) onto thetriangular nanoframes of Example 1, causing the walls to thicken and thecentral pore to shrink (FIG. 1, Step B). Subsequent additions of HAuCl₄followed by L-ascorbic acid (FIG. 1, Steps A and B), were performed toprogressively reduce the size of the triangular nanoframe pore.

In a typical experiment, an excess of L-ascorbic acid (1 mL, 5 mM) wasadded dropwise to a rapidly stirring colloid of two-component nanoframes(50 mL, Au:Ag=1:9 nanoframes). After addition of the reducing agent, thepale blue colloid gradually became turquoise, as evidenced by anincrease in intensity accompanied by a blue-shift in the absorption bandfrom 775 nm to 650 nm in the UV-visible spectrum of the solution (FIG.3A). Growth of a second band centered at 463 nm was also observed. Theobserved change in the UV-visible spectrum is consistent with silverions, generated from the initial etching process with gold, beingreduced back onto the nanoframe. A second aliquot of HAuCl₄ (22 μL, 5mM) was added dropwise, followed by more L-ascorbic acid (1 mL, 5 mM).The surface plasmon bands associated with the partially filledtriangles, at 650 nm and 463 nm, red-shifted to 665 nm and 480 nm,respectively (FIG. 3B). The third and final addition of HAuCl₄ (22 μL, 5mM) and L-ascorbic acid (1 mL, 5 mM) caused the most intense band at 665nm to further red-shift to 693 nm (FIG. 2C). The shorter wavelength bandalso red-shifted to 508 nm but became a shoulder on the main surfaceplasmon band. The red-shift observed after the second and thirdHAuCl₄/L-ascorbic acid additions was consistent with gold ions beingreduced onto the nanoframe. Reduction of gold ions onto the nanoframewalls is responsible for the “backfilling” of the nanoframes to formalloy nanoprisms. The change in pore size of the triangular nanoframesas a function of Au deposition was monitored by TEM (FIG. 3 D-F). Afterthe first reduction, the pore size decreased from about 33 nm (σ=23%,N=286) to about 14 nm (σ=16%, N=695). The second addition of gold saltfollowed by reduction, generated triangular nanoframes with average poresizes of about 7 nm (σ=14%, N=744). After the third gold/reductioncycle, many of the nanoframes were completely filled, and the remainingparticles possessed average pore sizes of about 4 nm (σ=13%, N=659).After one cycle of HAuCl₄/L-ascorbic acid, the thickness of thenanoframes increased from approximately 10 nm (σ=13%) to 12.4 nm (σ=11%,N=89). After three HAuCl₄/L-ascorbic acid cycles, the thickness hadincreased to about 15.2 nm (σ=10%, N=8). The UV-vis spectrum of thefilled nanoframes was red-shifted and dampened with respect to the puresilver nanoprisms (FIG. 3C). TEM-Energy Dispersive X-ray (EDX) analysisconfirmed that the back-filled nanoprisms were gold-silver alloys. Theaverage edge length of the gold-silver alloy nanoprisms is approximately63 nm.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiment describedhereinabove is further intended to explain the best mode known forpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with variousmodifications required by the particular applications or uses of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

1.-22. (canceled)
 23. A triangular nanoframe having an edge length ofless than about 200 nm and a thickness of less than about 100 nm and apore through the center of the {111} face of the nanoframe, wherein thenanoframe is free of pores through the {110} faces.
 24. The triangularnanoframe of claim 23, wherein the edge length is between about 70 nmand about 80 nm.
 25. The triangular nanoframe of claim 23, wherein thethickness is between about 5 nm and about 15 nm.
 26. The triangularnanoframe of claim 23, wherein the nanoframe is a metal nanoprism. 27.The triangular nanoframe of claim 26, wherein the metal is silver. 28.The triangular nanoframe of claim 23, wherein the pore size is betweenabout 5 nm and about 35 nm.
 29. A triangular nanoframe made by theprocess comprising etching a nanoprism with a salt to form ananotriangle.
 30. The triangular nanoframe of claim 29, wherein thenanoprism is a silver nanoprism and the salt is HAuCl₄.
 31. Thetriangular nanoframe of claim 23, wherein the pore is backfilled. 32.The triangular nanoframe of claim 31, wherein the pore is backfilledwith a metal.
 33. The triangular nanoframe of claim 32, wherein themetal is silver.
 34. The triangular nanoframe of claim 32, wherein themetal is gold.